In production of semiconductor devices, an electron beam exposure technique receives a great deal of attention as a promising candidate of lithography capable of micro-pattern exposure at a line width of 0.1 μm or less. There are several electron beam exposure methods. An example is a variable rectangular beam method of drawing a pattern with one stroke. This method suffers many problems as a mass-production exposure apparatus because of a low throughput. To attain a high throughput, there is proposed a pattern projection method of reducing and transferring a pattern formed on a stencil mask. This method is advantageous to a simple repetitive pattern but disadvantageous to a random pattern such as a logic interconnection pattern in terms of the throughput, and a low productivity disables practical application.
To the contrary, a multi-beam system for drawing a pattern simultaneously with a plurality of electron beams without using any mask has been proposed and is very advantageous to practical use because of the absence of physical mask formation and exchange. What is important in using a multi-electron beams is the number of electron lenses formed in an array used in this system. The number of electron lenses determines the number of beams, and is a main factor which determines the throughput. Downsizing the electron lenses while improving the performance of them is one of the keys to improving the performance of the multi-beam exposure apparatus.
Electron lenses are classified into electromagnetic and electrostatic types. The electrostatic electron lens type does not require any coil core or the like, is simpler in structure than the electromagnetic electron lens type, and is more advantageous to downsizing. The principal prior art concerning downsizing of the electrostatic electron lens (electrostatic lens) will be described.
A. D. Feinerman et al. (J. Vac. Sci. Technol. AlO(4), p. 611, 1992) disclose a three-dimensional structure made up of three electrodes as a single electrostatic lens by a micromechanics technique using a V-groove formed by a fiber and Si crystal anisotropic etching. The Si film has a membrane frame, membrane, and aperture formed in the membrane so as to transmit an electron beam. K. Y. Lee et al. (J. Vac. Sci. Technol. B12(6), p. 3,425, 1994) disclose a multilayered structure of Si and Pyrex glass fabricated by using anodic bonding. This technique fabricates microcolumn electron lenses aligned at a high precision. Similar to the previous reference, this Si film also has a membrane frame, membrane, and aperture formed in the membrane.
As a method of arraying downsized electron lenses, several arrangements have been proposed. T. H. P. Chang et al. (J. Vac. Sci. Technol. B10 (6), p. 2,743, 1992) disclose an arrangement of units each formed from one small electrostatic lens. This arrangement requires the wiring line of each lens and the support of each unit, which increases the volume and inhibits an increase in the number of arrays. The electron beam exposure technique is applied to micro-pattern exposure at a line width of 0.1 μm or less. The positional precision of each unit must be suppressed to an error of 0.1 μm or less, which makes mounting and assembly difficult and leads to an increase in apparatus cost. In the use of a plurality of electron sources, the exposure amount varies owing to variations between the electron sources, and the resolution of a developed resist pattern varies in a plane.
According to another proposal, electron lenses are arrayed on one Si substrate whose periphery is fixed. G. W. Jones et al. (J. Vac. Sci. Technol. B6 (6), p. 2,023, 1988) propose electron lenses one-dimensionally aligned on an Si substrate. U.S. Pat. No. 4,419,580 proposes electron lenses two-dimensionally arrayed on an Si substrate. This arrangement has merits that only Si substrates having electrodes are aligned instead of aligning units, and that the number of arrays can be easily increased by arranging a plurality of electrodes for electron lenses on one substrate. However, this arrangement suffers the following problems.
(1) A stress is applied to a substrate having a plurality of apertures in supporting the substrate from its side surface, and the substrate may warp due to the stress. An excessively large stress may destruct a thin membrane portion having apertures. When the same voltage is applied to an array, a generated electrolytic potential differs depending on the position of a lens in the array owing to the warp, resulting in variations in lens performance.
(2) For example, in a single electrostatic lens, a voltage is applied to a central electrode (second electrode), the first electrode on the incident side and the third electrode on the beam exit side are grounded, and a convex lens is formed from a composite lens of concave, convex, and concave lenses. In this single electrostatic lens, the gaps between the central electrode and the first and third electrodes must be minimized to realize a high lens efficiency, i.e., short focal length. If the electrodes are set closer, the membrane may warp due to electrostatic attraction generated by an applied high voltage of several kV. A generated electrolytic potential changes depending on the position of a lens in the array owing to the warp, causing variations in lens performance.
(3) If the membrane warps, and the electrodes come closer, discharge readily occurs at a position where the distance between facing electrodes is the shortest. In the worst case, discharge may destruct the electrodes.